The present invention relates to pulse or function generation, and more particularly to a triggered direct digital synthesis (DDS) pulse generator architecture that produces a pulse or other waveform at a fixed time after receiving a trigger signal while maintaining an accurate and stable output frequency.
An advantage of using DDS is that the output frequency may be very accurate and stable. The output frequency is determined by a phase accumulator and a DDS clock. The timing error in the phase accumulator circuit is generally much less than the error in the DDS clock. The DDS clock is usually derived from a phase locked loop (PLL) which is referenced to a crystal controlled oscillator (VXCO). This allows the DDS clock to be relatively accurate and stable, allowing the frequency of the signal being generated to be accurate and stable.
Pulse generators have usually had a triggered mode of operation. In this mode an output pulse is generated after receiving a trigger signal at an input. One way to provide a triggered operation using DDS is to start the generation of the output pulse on the next DDS clock after receiving the trigger signal. This leads to a time uncertainty of up to one DDS clock period. For many applications this is unacceptable. Another way to provide triggered operation is for the DDS clock to stop after producing an output pulse and wait for the trigger signal. When the trigger signal arrives, the DDS clock is started and a new output pulse is produced. This type of clock is sometimes referred to as a gated clock. The primary disadvantage of this scheme is that it is difficult to phase lock an oscillator to a reference clock when the oscillator is starting and stopping. Also gated oscillators typically have some frequency variability as they start. To have both accurate operation and triggered operation a DDS generator may have two clocks. One is a phase locked oscillator that is accurate, but cannot be used in the triggered mode. The other clock is not very accurate, but may be stopped and started to allow the triggered mode. Such a DDS generator has a much reduced frequency accuracy specification when operated in the triggered mode.
What is desired is to generate an output pulse at a fixed time after receiving a trigger while maintaining an accurate and stable frequency.